1. Field of the Invention
The present invention relates to a developing device used in a copying machine, a facsimile device, a printer and the like, and an image forming apparatus and a process cartridge that use the developing device.
2. Description of the Related Art
There has conventionally been widely used an image forming apparatus with a developing device using a two-component developer composed of toner and magnetic carrier. As this type of image forming apparatus, there is an image forming apparatus in which toner is replenished, according to need, from a toner container to a developer contained in a developing device that consumes toner as it conducts development, and thereby maintains the toner density of the developer within a predetermined range. In such a configuration, because the carrier within the developer is repeatedly used without being consumed significantly, the carrier degrades as an image is output. Specifically, the coated layer on the surface of the carrier is scraped off due to mechanical stress, and spent toner component is formed on the carrier surface. When the carrier degrades, the ability of the carrier to charge the toner decreases gradually, causing not only abnormal images such as scumming, image density reduction and image density irregularity, but also toner scattering. Therefore, a serviceman is sent to the user of this type of image forming apparatus to replace the carrier regularly. For this reason, the maintenance cost and the cost per image formation increase.
Japanese Patent Application No. 2891845 and Japanese Unexamined Patent Application Publication No. 2000-112238, for example, disclose a developing device in which a pre-mixed developer with a mixture of carrier and toner is replenished into a developer contained in the developing device in order to recover the toner density, and at the same time the increment of the developer is discharged from the developing device. In this configuration, old carrier is discharged little by little from the developing device by discharging the developer, and at the same time new carrier within the pre-mixed developer is replenished to the developer contained in the developing device. The carrier within the developer is replaced with the new carrier little by little by performing the above-described discharge and replenishment so that the carrier replacement work can be omitted.
However, the developer discharged from the developing device contains a larger proportion of degraded developer than the developer contained in the developing device. The degraded developer has a low fluidity, and thus there is a possibility that the developer firmly adheres to a conveying member that applies conveying force to the developer within a discharge conveyance path conveying the discharged developer. Specifically, when the conveying member that applies the conveying force to the developer in the conveyance path conveying the developer is a conveying screw, the developer having a low fluidity adheres and aggregates on a wing portion or an axis of the conveying screw, since the developer having a low fluidity has a high cohesive property.
When the developer has a high fluidity, the developer scatters easily in spite of its cohesive property, and the developer adhering to the conveying screw hardly aggregates and firmly adheres. When the developer has a low fluidity, on the other hand, the developer does not scatter once it aggregates, and thus the developer adhering to the conveying screw easily aggregates and firmly adheres. Because the developer adhering to the conveying screw only follows the rotation of the conveying screw, the rotation of the conveying screw does not apply the conveying screw to the developer. Moreover, when the fluidity of the developer to be conveyed is poor, the developer adhering firmly to the conveying screw aggregates gradually, reducing the conveying capability of the conveying screw.
Note that the problem in which the developer having a low fluidity adheres firmly to the conveying screw is not exclusive to the case where the conveying member is a screw, and thus the developer can firmly adhere to any conveying member that constantly applies a fixed conveying force to the developer.
In addition, when the developer firmly adheres to the conveying screw of the discharge conveyance path and thereby reduces the conveying performance of the developer within the conveyance path, the conveying capability of the discharge conveyance path drops in relation to the amount of developer to be discharged, which might clog up the discharge conveyance path. Also, a torque for rotating the conveying screw is increased by the firmly adhered developer, damaging the conveying member.
These problems are not exclusive to the developing device that uses a two-component developer composed of toner and magnetic carrier, and thus might occur in any developing device that uses a one-component developer, as long as such developing device is configured to discharge a developer by using developer discharge means.